The desire to decrease and ultimately eliminate dependence on fossil fuels has stimulated research into clean and renewable ways to produce electricity for the global marketplace. Solar power has become a viable option because it is a clean form of energy production and there is a potentially limitless supply of solar radiation. To that end, it is estimated the solar energy flux from the sun is approximately 2.7 megawatt-hours per square meter per year in certain advantageous areas of the world. With this tremendous amount of free and clean energy available, and the desire to reduce dependence on fossil fuels, solar power production is now, more than ever, being reviewed as an important means to help meet the energy consumption demands in various parts of the world.
Technological innovations and improvements have helped to make terrestrial solar power generation a feasible means for large scale power production. More specifically, the reduction in the magnitude of capital investment required and the reduction in recurring operation and maintenance costs allow solar power generation to compete with other forms of terrestrial power generation. Further, the scalability of solar power plants has the potential to enable smaller facilities to be constructed, with production capacity on the order of ten kilowatts, for communities with smaller demands, and larger facilities, capable of producing one hundred megawatts or more, for large metropolitan areas with higher energy demands.
To address the above demand for solar power systems many configurations have been designed and implemented. One such implementation is a concentrated solar power system that collects solar energy and concentrates that energy onto an absorber. The absorbed optical energy is carried away from the absorber by a fluid, for example molten salt, and then pumped to a power conversion system. The power conversion system then produces electricity that is eventually fed into the national electrical grid. After the fluid leaves the power conversion system it is then pumped back to the absorber.
A typical concentrated solar power system uses a fluid to transport absorbed heat energy from a heat receiver to a heat-to-electricity conversion system. A fluid with significant thermal capacitance, typically molten salt, is used to allow storing collected energy as sensible heat in the fluid. The ability to store energy allows separating the energy collection and energy production functions so that energy can be produced during periods of high demand, even nighttime, while energy collection is conducted when sufficient sunlight is available. This significantly enhances the economics of the power plant. The energy collection typically includes a central receiver/absorber surrounded by a large field of heliostats. The central receiver is typically a tall cylindrical tower made up of multiple absorber tubes. The heliostats intercept the incident solar energy and reflect it to the absorber tubes making up the receiver tower. The reflected energy is absorbed on the absorber tubes while molten salt flowing on the inside of the tubes is used to transport the absorbed energy effectively cooling the absorber tubes. The energy contained in the molten salt, as sensible heat, can then be used to drive a heat engine. Although this system has the advantage of thermal energy storage via the molten salt, the system has low energy collection efficiency due to inefficiencies in the heliostat optical system and from heat losses off the large open-air receiver/absorber. Conversely, point focus solar power systems, typically using a parabolic dish concentrator coupled to an absorber cavity, have high solar energy collection efficiency and are capable of achieving higher temperatures. However, typical implementation of this system provides direct conversion of the absorbed energy to electricity via a thermal engine, for example a Stirling engine, coupled directly to the absorber cavity. There is no energy storage capability, therefore, the economics of this system suffer because the energy production cannot be optimized to follow the energy demand.
Accordingly, a need exists for a solar power generation system capable of efficient energy collection, with high temperature capability, and with the ability to store collected energy so that electrical energy production can be optimized to follow periods of high power demand.